1. Field of the Invention
The present invention relates generally to electrochemical cell components and structures. More particularly, the invention relates to composite solid electrolytes and methods to fabricate composite solid electrolytes, as well as protected anodes and battery cells incorporating the composite solid electrolytes.
2. Related Art
The low equivalent weight of alkali metals, such as lithium, render them particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
An important challenge in the development of rechargeable lithium metal batteries is effective cell cycling. On repeated charge and discharge cycles, lithium “dendrites” gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow “mossy” deposits that can dislodge from the negative electrode and thereby reduce the battery's capacity.
To address lithium's poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a “protective layer.” Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying protective layers have not succeeded.
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell's electrolyte that contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various pre-formed lithium protective layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (“LiPON”) or related material. LiPON is a glassy single ion conductor (conducts lithium ion) that has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
Work in the present applicants' laboratories has developed technology for the use of glassy or amorphous protective layers, such as LiPON, in active metal battery electrodes. (See, for example, U.S. Pat. No. 6,025,094, issued Feb. 15, 2000, U.S. Pat. No. 6,402,795, issued Jun. 11, 2002, U.S. Pat. No. 6,214,061, issued Apr. 10, 2001 and U.S. Pat. No. 6,413,284, issued Jul. 2, 2002, all assigned to PolyPlus Battery Company).
Prior attempts to use lithium anodes in corrosive environments have met with difficulty. Lithium anodes in aqueous environments, for example, relied either on the use of very basic conditions such as use of concentrated aqueous KOH to slow down the corrosion of the Li electrode, or on the use of polymeric coatings on the Li electrode to impede the diffusion of water to the Li electrode surface. In all cases however, there was substantial reaction of the alkali metal electrode with water. In this regard, the prior art teaches that the use of aqueous cathodes or electrolytes with Li-metal anodes is not possible since the breakdown voltage for water is about 1.2 V and a Li/water cell can have a voltage of about 3.0 V. Direct contact between lithium metal and aqueous solutions results in violent parasitic chemical reaction and corrosion of the lithium electrode for no useful purpose. Thus, the focus of research in the lithium metal battery field has been squarely on the development of effective non-aqueous (mostly organic) electrolyte systems.